The present invention is directed to a method for blackening ferrous materials and to an improved blackening solution and in particular to a blackening solution which will cause only negligible precipitation by reaction with the workpiece that is being coated.
Black coatings are formed on a ferrous metal by immersing the ferrous metal object in a blackening solution bath. One type of known blackening solution is designed to be used at room temperature and is primarily an aqueous formulation. A typical room temperature blackening solution formulation contains: selenium dioxide, an inorganic copper salt such as copper sulfate or copper nitrate, and an inorganic acid such as phosphoric, sulfuric, nitric or hydrochloric acid.
An inherent drawback in the known selenium-copper room temperature blackening solutions is that a precipitate forms in the working solution as the bath or solution is used. Some of the iron in the workpiece dissolves in the solution. The dissolved iron reacts with the selenium ions present in the bath to form an iron-selenium or iron-selenium phosphate complex. This causes the bath to become self-consuming. With time, and as more work is passed through the bath, more iron is dissolved and more selenium is precipitated in an unproductive manner. Eventually, the chemical balance of the bath changes to such a degree that individual control of several bath parameters such as dissolved metal concentration (selenium and copper) and acid concentration (phosphoric, nitrate, sulfuric or hydrochloric) is needed. This is both time-consuming and expensive.
In addition, the selenium-iron or selenium phosphate precipitate is a toxic substance. This means that special handling and disposal procedures must be observed.
Accordingly, it is an object of the present invention to provide a room temperature blackening solution which does not suffer from the above described drawbacks.
It is also an object of the present invention to provide a room temperature blackening solution for ferrous metals that produces only negligible by-product precipitation of the active ingredient selenium.